This invention relates to a method and apparatus for concentrating high molecular weight materials in an aqueous medium or for harvesting cells from an aqueous medium.
There is available at the present time, a device for concentrating high molecular weight materials in an aqueous medium by utilizing ultrafiltration membranes arranged in a configuration which effects tangential flow across the membranes while allowing the desired separation to occur. Such a device is available from Millipore Corporation and identified as the Pellicon.RTM. cassette system. The assembly contains a plurality of repeating units each of which consists of two separator screens and two filters alternately layered. Entrance and exit ports are provided that permit tangential flow across the filter surface and permit separate recovery of a filtrate and a retentate. A plurality of repeating units are maintained in the desired configuration by being sandwiched between an upper and a lower gasket and held together by pressure. The screens which contact both surfaces of each filter function to provide a relatively open pathway for incoming aqueous medium and for the filtrate so that the incoming aqueous medium passes tangentially over the filter surface while allowing the filtrate to pass through the membrane and out of the assembly and allowing the retentate to be removed from the assembly by means separate from the filtrate output means. In addition, the presently available separation assembly provides only parallel flow of the incoming aqueous medium simultaneously across plurality of filter surfaces.
While presently available separation assemblies have been satisfactory for most purpose, it has found that the screens in the assembly which contact the incoming aqueous medium can become clogged with particulate matter. The resultant blockage requires that the pressure of the incoming stream be increased to compensate for the pressure drop caused by the entrapped particles. The increased pressure can cause malfunction of the assembly due to separation of the assembly elements or rupture of the filter layer. In addition, the use of the screen to separate filter layers does not permit adjustment of the open space between adjacent filter layers so that the wall shear forces on the incoming liquid passing through the space between the two filter layers can not be adjusted. In addition, tangential flow of incoming liquid into a single space between two adjacent filter layers, as in the case of present parallel flow devices, does not permit substantially complete filtration of the incoming liquid due to the short residence time of the incoming liquid in contact with the filter layer. In contrast, it would be desirable to provide a means for easily controlling the wall shear forces on incoming liquid while preventing entrapment of the particles in the incoming liquid. Furthermore, it would be desirable to provide such a means which permits incoming liquid to serially contact a plurality of filter layers.